1. Field of the Invention
This invention relates to data telemetry apparatus and methods for oilfield wellbore operations.
2. Description of the Related Art
A variety of communication and transmission techniques have been used to provide real time data from the vicinity of a drill bit to the surface during drilling. The utilization of measurement-while-drilling (MWD) sensors with real time data transmission provides substantial benefits during a drilling operation. For example, continuous monitoring of downhole conditions allows for a prompt response to potential well control problems and results in improved drilling efficiency and hole cleaning.
MWD systems provide drilling operators greater control over the construction of a well by providing information about conditions at the bottom of a wellbore substantially in real time as the wellbore is being drilled. Certain information is of interest to drilling operators, and is preferably obtained from the bottom of the wellbore substantially in real time. This information commonly includes directional drilling variables such as inclination and direction (azimuth) of the drill bit, and geological formation data, such as natural gamma ray radiation levels and electrical resistivity of the rock formation. The term MWD system should be understood to encompass equipment and techniques for data transmission from within the well to the earth's surface.
Measurement of drilling parameters such as bit weight, torque, wear and bearing condition in real time provides for more efficient drilling operations. In fact, faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of a need to interrupt drilling for abnormal pressure detection is achievable using MWD techniques.
Common telemetry systems that have been used in an attempt to provide real-time data from the vicinity of the drill bit to the surface include mud pressure pulse systems, insulated conductor system, acoustic systems, and electromagnetic systems.
In a mud pressure pulse system, the resistance of mud flow through a drill string is modulated by means of a valve and control mechanism mounted in a drill collar near the bit and generates a pressure pulse that travels in the mud column to the surface. This type of system typically transmits data at low rates, typically less than 10 bits per second due to attenuation and distortion of the generated pulses.
An insulated conductor, or hard wire connection from MWD sensors to the surface, is an alternative method for establishing downhole communications. As used herein, the term insulated conductor means both electrical and optical conductors. This type of system is capable of a high data rate and high-speed two way communication is possible. This type of system may employ a special drill pipe and special tool joint connectors having the insulated conductors disposed therein. An alternative installation may use a cable within the pipe bore as the insulated conductor.
Acoustic systems have provided a third alternative. Typically, an acoustic signal is generated near the bit and is transmitted as stress waves through the wall of the drill pipe, or as pressure pulses or waves in the mud column. For acoustic signals transmitted as stress waves through the walls of the pipe, reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints results in a reduced signal bandwidth. In addition, contact between the drill pipe and the borehole wall, such as may occur, for example, in a directional well, results in a very high level of signal attenuation that makes signal detection difficult at the surface.
The fourth technique used to telemeter downhole data to the surface uses the transmission of electromagnetic waves through the earth. A current carrying downhole data signal is input to a toroid or collar positioned adjacent to the drill bit or input directly to the drill string across an electrical isolator. When a toroid is utilized, a primary winding carrying the data for transmission, is wrapped around the toroid and a secondary winding is formed by the drill pipe. A receiver is connected to the ground at the surface where the electromagnetic data is picked up and recorded. It has been found, however, that in deep or noisy well applications, conventional electromagnetic systems experience difficulty in extracting the data signal from the ambient noise at the surface. The surface noise includes telluric noise as well as manmade noise from electric machinery and generators associated with the drilling process. In addition, electromagnetic noise may be generated by the movement of the conductive drill string in the wellbore. In addition, the attenuation of electromagnetic waves above about 20 Hz is extreme, resulting in very small signal at the surface. It is common that the noise source signals are significantly greater than the desired transmitted signals. While much of the noise can be removed from the surface measurements, the high attenuation and low transmission bandwidth limit the use of common electromagnetic techniques to relatively shallow depths and/or low bit rates.
All of the systems mentioned above employ some type of signal acquisition at the surface. In many cases, the presence of a rotating drill string makes optimal placement of the detection sensors a problem. For example, pressure pulse signals are commonly detected by a pressure transducer mounted upstream (closer to the pump) of a Kelly hose on a non-rotating portion of the fluid supply line. However, this location makes detection more difficult due to pressure pulse signal attenuation due to the compliant Kelly hose and due to reflections from pipe connections. Mounting of the pressure transducer on the rotating drill string, for example above the Kelly joint and before the Kelly hose, or within the rotating portion of a top drive, can provide superior detection. However, the transfer of the signal from the rotating framework to the stationary rig environment requires slip-rings or inductive couplers. Likewise, the use of hard-wired drill string connections, as described above, commonly requires slip rings or inductive couplers mounted on the Kelly to transfer the signal from the rotating to non-rotating environment, and vice versa. In addition, when a slip-ring or inductive coupler is used to transfer the signal from the rotating member to the stationary rig environment, a cable is commonly run through the top drive and along the Kelly hose to connect the slip-ring or inductive coupler with a surface controller for both signal and power transfer. These cables can pose repair and maintenance problems. Thus, there is a need for an improved surface telemetry system for use during wellbore operations.